1. Field of the Invention
This invention relates to non-volatile semiconductor memory devices.
2. Description of the Related Art
Currently available large-scale semiconductor memory devices are typically designed to come with built-in redundancy circuitry which permits relieving the device including defects, if any. The same goes with electrically erasable programmable read-only memory (EEPROM) chips. With standard redundancy circuit designs, a redundant row cell array and redundant column cell array are provided in addition to an ordinary or standard cell array. Also provided is fuse circuitry which stores therein defect addresses and detects whether an externally supplied address coincides with the presently stored defect address for controlling address replacement or substitution. The fuse circuitry is typically designed to employ an array of laser-blown type fuses.
The fuse circuit is also used to write or “program” a variety of kinds of initial setup data for determination of the operating conditions of a memory chip, in addition to defect address data for defect repairing purposes. Examples of such initial setup data include, but not limited to, data for adjustment of voltages as internally produced on a chip in accordance with fabrication process parameter irregularities, setup data of a write voltage(s), and control parameters of the requisite number of write/erase control loops.
Unfortunately, the fuse circuit lacks functional flexibilities due to the fact that it will hardly permit any re-programming once after having programmed in a certain way. Another disadvantage is that extraction of defective portions through a test at the stage of wafer test/inspection and laser blowout process must be done separately at different process steps. Additionally these are hardly implementable as a series of continuous steps.
A currently proposed approach to avoiding the problems is to employ as an initial setup data storage circuit in place of the fuse elements an array of electrically rewritable non-volatile memory cells which are the same as those used in EEPROMs. Employing such non-volatile memory cells makes data writing easier when compared to traditional fuse blowout techniques, while enabling establishment of rewriting or “reprogramming” of the data.
However, one prior known scheme is for disposing the non-volatile memory cell array for storage of the initial setup data in a specific region that is kept separate from a “main” memory cell array. With this scheme, storing initial setup data does require the use of separate or “extra” rewrite/read circuitry in addition to the main memory cell array's inherent rewrite/read circuitry, which would result in an increase in complexity of circuit configuration while increasing the resultant chip area. Another problem faced with the prior art is the difficulty in operation control procedure due to the necessity of verification and correction after once having written the initial setup data.